Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and a rotor. The rotor typically includes a rotatable hub having one or more rotor blades attached thereto. A pitch bearing is typically configured operably between the hub and a blade root of the rotor blade to allow for rotation about a pitch axis. The rotor blades capture kinetic energy of wind using known airfoil principles. The rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
The amount of power that may be produced by a wind turbine is typically limited by structural limitations (i.e. design loads) of the individual wind turbine components. For example, the blade root of a wind turbine may experience loads (e.g. a blade root bending moment) associated with both average loading due to turbine operation and dynamically fluctuating loads due to environmental conditions. Such loading may damage the pitch bearing, thereby eventually causing the pitch bearing to fail. The fluctuating loads can change day-to-day or season-to-season and may be based on wind speed, wind peaks, wind turbulence, wind shear, changes in wind direction, density in the air, yaw misalignment, upflow, or similar. Specifically, for example, loads experienced by a wind turbine may vary with wind speed.
As such, it is imperative to monitor loads acting on the wind turbine to ensure design loads are not exceeded. Various systems and methods have been employed to estimate loads experienced by a wind turbine. For example, one system estimates loads by determining a thrust acting on the wind turbine. The terms “thrust,” “thrust value,” “thrust parameter” or similar as used herein are meant to encompass a force acting on the wind turbine due to the wind. The thrust force comes from a change in pressure as the wind passes the wind turbine and slows down. For example, FIGS. 1 and 2 illustrate a loading and a thrust acting on a wind turbine component as a function of wind speed, respectively. The solid lines represent an average load and an average thrust for three different turbulence intensity levels, whereas the dotted lines represent a maximum load and a maximum thrust, respectively. More specifically, solid lines 100, 200 and dotted lines 102, 202 represent a rough wind day with approximately 25% turbulence intensity; solid lines 104, 204 and dotted lines 106, 206 represent an intermediate wind day with approximately 15% turbulence intensity; and solid lines 108, 208 and dotted lines 110, 210 represent a relatively smooth wind day with approximately 5% turbulence intensity.
As shown in FIG. 1, the average load is almost the same for all three turbulence intensity levels; however, the maximum load dramatically increases with increased turbulence intensity and wind speed. Similarly, as shown in FIG. 2, the corresponding thrust also dramatically increases with increased turbulence intensity and wind speed. In other words, there is a direct correlation between wind turbine loading and thrust, as shown in FIG. 3. Accordingly, estimating the thrust experienced by the wind turbine may assist in predicting, and therefore minimizing, loads acting on various wind turbine components.
Further control strategies have utilized various control technologies that utilize algorithms to estimate loads acting on a wind turbine. For example, referring now to FIG. 4, a wind turbine implementing one such control technology estimates loads acting on the wind turbine by determining an estimated thrust (line 402). The technology calculates the estimated thrust 402 using a plurality of turbine operating conditions, such as, for example, pitch angle, power output, generator speed, and air density. The operating conditions are inputs for the algorithm, which includes a series of equations, one or more aerodynamic performance maps, and one or more look-up tables (LUTs). In the illustrated embodiment, for example, the LUT is representative of a wind turbine thrust. A +/−standard deviation of the estimated thrust (lines 404, 406) may also be calculated, along with an operational maximum thrust (point 408) and a thrust limit 410. As such, the wind turbine may be controlled based on a difference between the maximum thrust 408 and the thrust limit 410. Such control technologies, however, are typically only representative of ideal operating conditions (i.e. steady state conditions) and do not represent dynamic or fluctuating loads due to environmental conditions. Additionally, the thrust limit 410 remains constant even though loads experienced by the wind turbine may vary with wind speed.
Accordingly, an improved system and method for preventing excessive loads on a wind turbine that varies the thrust limit would be welcomed in the technology. More specifically, a system and method that incorporated dynamically fluctuating loads due to environmental conditions would be advantageous.